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Green Sand Metalcasting Foundry News

Predicting, Preventing Core Gas Defects in Steel Castings

Posted by Hill and Griffith Company on Jan 28, 2020 8:00:00 AM

Modeling and analyzing core gas evolution and metal solidification behavior can aid in the prediction and prevention of porosity caused by core gas.

Article excerpt from the Modern Casting September 2014 issue by L. Xue, M. Carter, A. Catalina, Z. Lin, C. Qiu and C. Li.

Porosity is a common but serious casting defect. One type of porosity is a result of core gas that has evolved and become trapped in the casting during solidification. Detailed information is needed to reduce or eliminate core gas-related defects regarding the core gas generation, flow and venting in the core, and the metal flow and solidification behavior in the mold. In a recent study, numerical simulations were conducted based on a prototype design for a steel casting for Caterpillar. Core gas and porosity defects calculated in the simulations were analyzed and compared with the real casting results.

The gases dissolved during solidification can be caused by hydrogen or nitrogen in the initial liquid or core gas decomposed from the sand core and vented to the liquid, and they play a major role in porosity formation in castings. For all the analytical models developed to predict porosity defects in castings, most are based on tracking the evolution of dissolved gases in the initial liquid. Due to the complicated physics involved, modeling the core gas evolution in castings is difficult. However, without the consideration of core gas, predictions of porosity defects are insufficient.

Preventing Porosity 1

For example, in a previous study, porosity defects in a steel casting were predicted. One of the three main regions that showed porosity in the actual casting was missed in the simulation. The missed porosity region might have resulted from core gases. As the quality requirements of parts become more stringent, the ability to precisely predict defects, including core gas-related defects, becomes more important.

Simulating the casting process involves a wide variety of models, such as fluid dynamics, heat transfer and solidification. A general-purpose computational fluid dynamics package might study gas generation and flow and venting in the core but typically will not track the core gas evolution in the liquid metal. By analyzing the core gas venting, metal flow and solidification behavior, possible core gas defects in the castings still can be extrapolated.

Solidification Macroshrinkage

Two typical models can be employed to predict macroporosity formation in metals due to shrinkage. A hydrodynamic model can predict the evolution of velocity and pressure in the solidifying metal. Despite being an accurate tool to study the porosity formation phenomena, this model may be computationally costly because at each time step, the numerical algorithm involves the complete solution of momentum and energy equations. The time step, controlled by various stability criteria associated with fluid flow, also may be short compared to the total solidification time of the casting. The latter may be as long as hours for large sand castings.

Another shrinkage model based on only the solution of the metal and mold energy equations (not fluid flow equations) can predict porosity by evaluating the volume of the solidification shrinkage in each isolated liquid region at each time step. This volume then is subtracted from the top of the liquid region in accordance with the amount of liquid metal available in the cells from which the fluid is removed. The direction of gravity defines the top of a liquid region. The relevance of this approach is supported by the fact that in many situations, fluid flow in the solidifying metal can be ignored. Porosity formation in that case is primarily governed by metal cooling and gravity. Feeding due to gravity often occurs on a time scale much shorter than the total solidification time.

Microshrinkage

Preventing Porosity 2Preventing Porosity 3

Illustrations show the porosity defects on the middle-plane longitudinal cross section of the steel casting: a) porosity defects on the test casting; b) porosity by the rapid solidification model; and c) predicted microporosity by the first principles model.

Core Gas

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